1. Field of the Invention
The invention relates to the field of infrared and terahertz bolometric detection and, more especially, the field of bolometric detection using an array of micro-plates that are suspended above a substrate.
2. Description of Related Art
It is frequently acknowledged that infrared detection, i.e. detection in the wavelength range from 0.75 μm to 1,000 μm, is a technological field that is fraught with particular problems. In fact, every object emits in the infrared spectrum as soon as its temperature exceeds 0° K. Thus, if an infrared detector is not cooled, the devices that surround the sensitive elements (substrates, connectors and wiring, packages, optics, etc.) emit significant infrared radiation which is added to the radiation originating from the scene that one is attempting to detect. This unwanted component can be very considerable and sometimes constitutes more than 99% of the total signal produced by the detection elements at a temperature of 300° K. This unwanted component is commonly referred to as “thermal noise” or “common mode noise”.
Consequently and in contrast to other types of detection, especially detection in the visible spectrum, there is a need to provide structures and operating principles that are capable of effectively managing this common mode noise. To achieve this, the first high-sensitivity infrared detectors were cooled to extremely low temperatures of around a hundred degrees Kelvin or even several degree's Kelvin in order to minimize common mode noise.
Also, there are two distinct classes of infrared detectors, namely “quantum” detectors and “thermal” detectors, especially thermal bolometric detectors. It is also well known that the physical principles used by these two types of detection are fundamentally different and that each have their own problems.
In the case of quantum detectors, a semiconductor is used to produce electron-hole pairs due to the effect of photon absorption in the infrared spectrum with the charge carriers thus created being collected via electrodes which are usually combined with a PN type junction.
In contrast, in the case of bolometric detectors, an absorbent material that is selected for its ability to convert the power of the incident infrared flux into heat is used. This material, or a second material that is in contact with the first material, is also used to convert the heat produced into a variation of an electrical characteristic, generally speaking a variation in electrical resistance. This variation of the electrical characteristic is then measured.
One particular bolometric detector architecture has been devised in order to manage common mode noise, namely a detector that comprises an array of bolometric micro-plates that are suspended above a so-called “readout” substrate by means of support and thermal isolation arms.
As known in itself, this architecture is specifically provided to thermally isolate the bolometric elements from the substrate, which is the main source of common mode noise because it is located extremely close to them. This produces, firstly, a significant gain in terms of sensitivity and, secondly, this architecture also makes it possible to do away with the need for cooling down to extremely low temperatures.
Although an architecture based on suspended micro-plates has many advantages, especially the possibility of being used without being cooled down to extremely low temperatures, the presence of the support arms of the bolometric micro-plates makes it impossible to achieve a satisfactory fill factor using current fabrication techniques—the more the micro-plates are miniaturized, the worse the fill factor becomes.
Solutions have been developed in order to improve the fill factor. Nevertheless, these solutions make manufacturing processes more complex and involve higher costs. For example, Document U.S. Pat. No. 6,094,127 describes a detector with three superposed stages with, in particular, a stage that comprises an integrated circuit, a support stage and an absorption stage. The absorption stage can thus occupy the entire surface area of the detector, thereby improving its efficiency. However, in order to electrically connect the absorption stage to the support stage, an electrical interconnection element is interposed between the support and absorption stages. This electrical interconnection element consists of a conductive channel enclosed in a dielectric sheath. This results in a complex manufacturing process which poses a risk to electrical continuity from one stage of the detector to another; this continuity is, however, a crucial element for ensuring optimal operation of the detector. In addition, the presence of the electrical interconnection element that is in contact with the absorption stage can have an adverse impact on the absorption quality and sensitivity of the detector.
Also, in order to improve the efficiency of detectors and/or to reduce manufacturing costs, batch-processing fabrication methods are usually used with several arrays of micro-plates being manufactured jointly from a single silicon wafer and then being individualized as described, for instance, in Documents U.S. Pat. Nos. 6,753,526 and 6,924,485.
Given the fact that batch-processing fabrication methods are already employed in order to manufacture the arrays of micro-plates, batch-processing fabrication methods originating from the microelectronics industry are also used to produce detectors that directly include vacuum packaging for every micro-plate, as described, for instance, in the above-mentioned documents. This packaging, which is commonly referred to as integrated hermetic micro-packaging, consists of a cap produced on top of each micro-plate that sits on the substrate on each side of the micro-plate and is hermetically vacuum sealed. Performing packaging steps in batch mode makes it possible to reduce the production time and production costs of detectors compared with a single hermetically sealed package that is individually made for each array of micro-plates.
However, the space that must be left between each micro-plate in order to support the caps results in a significant reduction in the optically active surface area of the detector for any given array size and hence a direct drop in the efficiency of the detector.
By virtue of its construction, the useful surface area of a bolometric micro-plate that is suspended by support arms and dedicated to detecting infrared or terahertz radiation is limited compared with the surface area of the substrate and this reduces the detector's sensitivity.
For example, producing detectors with square micro-plates having a side dimension of 12 μm, a size that currently reflects the maximum extent of miniaturization of bolometric micro-plates, that are absorbent around λ=10 μm requires a square substrate surface area having a side dimension of at least 17 μm for each micro-plate. The useful surface area of an array of micro-plates having a side dimension of 12 μm dedicated to detection therefore accounts for no more than 50% of the total surface area of the array.